The present invention relates generally to multi-heterodyne methods and systems, and more particularly to methods and systems for correcting phase and timing errors in multi-heterodyne signals.
A frequency comb is a broadband coherent source whose frequency spectrum can be fully described by two frequencies, namely, the offset and the repetition rate. Optical combs have found a variety of applications, e.g., in high precision metrology and spectroscopy. For example, in the terahertz frequency regime, combs generated by pulsed lasers can be useful sources of radiation for detecting molecular finger-prints, because many molecules have strong rotational and vibrational resonances in this frequency regime. Further, multi-heterodyne spectroscopy based on two frequency combs, which is also known as dual-comb spectroscopy, allows performing broadband spectroscopy with a broad spectral coverage, a high frequency resolution, and high signal-to-noise ratios. In dual-comb spectroscopy, two frequency combs are directed onto a common detector, and the heterodyne beating between different pairs of lines is detected.
The implementation of dual-comb spectroscopy can be, however, challenging because the carrier-phase drift of the combs can preclude coherent averaging. If the drift is known, its effect can be corrected. But measuring the absolute frequency of a comb line can be challenging. One approach for measuring the carrier-envelope offset (CEO) of a comb is to beat the comb with a stable continuous-wave (CW) laser. Another approach is to use a narrowband optical filter, such as a Bragg grating, to select only a portion of a comb's optical spectrum, and to extract the dual comb beating of different portions of the spectrum. Yet, another approach is to measure the CEO directly using a so-called f-2f technique. These conventional approaches, however, suffer from a number of shortcomings. In particular, they can require the use of additional lasers and optical components, or can impose certain requirements on the comb.
Moreover, performing dual-comb spectroscopy based on combs generated by quantum cascade lasers presents additional challenges. For example, the use of reference channels in long wavelengths for phase and timing correction can require additional cryogenically cooled optical detectors. In addition, the lasers themselves are typically cryogenically cooled in the long wavelength regime, and particularly in the terahertz (THz) regime. Thus, the use of additional CW lasers in reference channels can greatly increase the cost and complexity of a multi-heterodyne system.
Accordingly, there is a need for improved multi-heterodyne methods and systems, and more particularly, there is a need for improved methods and systems for processing multi-heterodyne signals.